[fix] proper rustc in devShell

[QuentinI]

Description

Small tweak to fix rustc in devShell not finding sysroot.

Linked issue

None

Benefits

Usable rustc in devShell

Checklist

• I've read CONTRIBUTING.md
• I've used the standard signed-off commit format (or will squash just before merging)
• All applicable CI checks pass (or I promised to make them pass later)
• (optional) I've written unit tests for the code changes
• I replied to all comments after code review, marking all implemented changes with thumbs up

[coveralls]

Pull Request Test Coverage Report for Build 6483708865

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 6380 unchanged lines in 107 files lost coverage.
• Overall coverage decreased (-3.4%) to 56.019%

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Files with Coverage Reduction	New Missed Lines	%
cli/src/main.rs	1	0.0%
cli/src/torii/pagination.rs	1	98.9%
config/src/torii.rs	1	96.43%
core/src/smartcontracts/isi/block.rs	1	87.5%
crypto/src/merkle.rs	1	96.23%
logger/src/lib.rs	1	95.15%
config/src/lib.rs	2	0.0%
logger/src/layer.rs	2	48.05%
cli/src/samples.rs	3	0.0%
data_model/derive/src/has_origin.rs	3	95.16%

Totals
Change from base Build 5423219773:	-3.4%
Covered Lines:	21933
Relevant Lines:	39153

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💛 - Coveralls